The predominant conventional method of measuring snow water-equivalent (SWE) is the fluid-filled snow pillow. Beaumont, R. T., Mt. Hood Pressure Pillow Snow Gage, Journal of Applied Meteorology, p. 626-631, October 1965. Engeset, R. V. et al., Snow Pillows: Use and Evaluation, Snow Engineering: Recent Advances and Developments, Hjorth-Hansen et al. Eds., Proceedings of the Fourth International Conference on Snow Engineering, Trondheim, Norway, pp. 45-51, 19-21 June 2000. Palmer, Peter L., Estimating Snow Course Water Equivalent from SNOTEL Pillow Telemetry: An Analysis of Accuracy, Western Snow Conference, Phoenix, Ariz., Phase Profilometry, 81-86, Apr. 15-17, 1986. Smith, T. W. and H. S. Boyne, Snow Pillow Behavior under Controlled Laboratory Conditions, 49th Western Snow Conference, Colorado State University, pp. 13-22, 1981.
Other methods of determining SWE include active and passive microwave techniques and measuring the attenuation of natural gamma rays that pass through the snow cover. Osterhuber, Randall et al., Snowpack Snow Water Equivalent Measurement Using the Attenuation of Cosmic Gamma Radiation, Western Snow Conference, Snowbird, Utah, April 1998. Ulaby, Fawwaz T. and William H. Stiles, The Active and Passive Microwave Response to Snow Parameters:2. Water Equivalent Dry Snow, Journal of Geophysical Research, Vol. 85, No. C2, pp. 1045-1049, Feb. 20, 1980.
The gamma and microwave methods have limited use at present and even as their use expands they will not replace ground based measurement that can measure SWE in real time. Microwave methods are generally used from satellites, thus they are limited to the period of time that the satellite is over the area of interest. Further, gamma attenuation methods require integration periods of up to four hours. A preferred embodiment of the present invention is an improvement over the microwave and gamma attenuation methods as it can be used to measure real time variations of moisture content, such as SWE, that are needed to forecast flood or landslide potential. Further, to fully appreciate the advantages of a preferred embodiment of the present invention, it is instructive to review the operation of an accepted conventional method of measuring snow water equivalent using a device termed a snow pillow.
The fluid-filled snow pillow is a bladder that is placed on the ground and subsequently filled with fluid, nominally a water-antifreeze mix. The change in pressure as snow accumulates on the bladder is used to determine SWE. The standard snow pillow is about three meters (m) (10 ft) in diameter. Smaller pillows are also used, but introduce larger measurement errors. The snow pillow was developed through a trial and error process that found that snow pillows with diameters smaller than about 3 m were subject to unexplained and inconsistent errors. The snow pillow at times may exhibit inconsistency in measurement. Even the 3 m snow pillows may provide inaccurate SWE measurements, primarily at the beginning of winter or at the transition between winter and spring. The cause of these errors was unknown until recently.
The inventors of the present invention determined that these measurement errors are caused primarily by the difference in thermal properties between the SWE detector and the soil upon which the detector is placed. A secondary influence is the mechanical property of the snow, i.e., elastic modulus and viscosity. Snow pillow measurement errors occur when snow load is shifted from the pillow to the surrounding soil through bridging or from the surrounding soil to the snow pillow. The errors may also occur because the amount of snow on the snow pillow is different than on the surrounding soil.
When thermal properties of a snow pillow, such as thermal conductivity and heat capacity, are different from the soil under and around it, the heat flux through the snow pillow will be different than through the soil. Acknowledging this fact is especially important during periods when the snow/soil interface is at the melting temperature. This difference in heat flux produces a difference in the snowmelt rate on the snow pillow compared to the surrounding soil, causing the snow load to transfer between the detector and the surrounding soil. The snow load will transfer to the snow pillow when the snowmelt rate over the pillow is less than that of the surrounding soil. The resulting error in measurement is termed an SWE over measurement. Conversely, the snow load will transfer from the snow pillow to the surrounding soil, i.e., an SWE under measurement, when the snowmelt rate over the pillow is greater than over the surrounding soil. Errors have the highest probability of occurrence in a) regions of deep snow cover, b) during unusual warming temperatures, or c) during spring.
Deep snow conditions cause the snow/soil interface to increase to the melting temperature by insulating the soil, thus reducing the conduction rate of stored heat in the soil. Warm air temperatures can produce an isothermal snow cover that is uniformly at the melting temperature, thus preventing heat from conducting from the soil, increasing the snowmelt rate at the snow/soil and snow/snow pillow interfaces. In the spring, when the snow temperature is isothermal, i.e., 0xc2x0 C., or active melting is occurring, the snow/soil interface will be at the melting temperature also.
Snow pillow measurement errors caused by an actual difference in the amount of snow on the pillow compared to the surrounding soil may occur because of a difference in heat flux between the detector and soil, i.e., analogous to the process that produces snow load shifting. Measurement errors may occur in the late autumn and early winter when falling snow melts at a different rate on the soil than on a snow pillow because the heat capacity of the soil is different from that of the snow pillow. The effects of differences in heat storage in the soil and snow pillow disappear once a stable snow cover forms and steady heat flux conditions are established.
A physical theory of SWE detector performance, developed by one of the inventors, indicates that increasing the diameter of an SWE detector decreases the errors caused by differences in heat flux through the detector as compared to the soil. Johnson, J. B., Interim Report on the 1997-98 and 1998-99 Field Test Performance of the CRREL-Electronic Snow Pressure Sensor and Proposal for Sensor Redesign, Ft. Wainwright, Ak., 1999. This is why a snow pillow must have a relatively large diameter compared to a moisture or soil stress detector of the present invention to achieve reasonable accuracy. However, increasing the detector diameter does not decrease errors caused by differences in the amount of snow melted over the detector compared to the ground.
Refer to FIG. 7 (Prior Art) depicting the SWE measurement performance of a 3 m snow pillow for a five-year period. The solid markers indicate manually measured SWE and the dashed curve represents snow pillow measurements. The snow pillow and manual measurements have a good agreement for most of the measurement period. However periods of disagreement between manual and snow pillow measurements occur in early Winter ""98. Differences are also evident during Winter/Spring ""01 and spring transition periods of 1998 and 2001. In general, the snow pillow accurately measures SWE. However, during 1998-99, the snow pillow over measures SWE during the early part. This may be the result of differences in heat capacity between the snow pillow and the soil as shown in FIG. 7A. Also, there are a number of periods when the snow pillow under measures SWE as shown in FIG. 7D for Winter/Spring ""01.
A number of U.S. patents propose various solutions to measuring SWE. U.S. Pat. No. 3,665,180, Method and Apparatus for Measuring the Water Content of a Snowpack, to Gullot et al., May 23, 1972 uses two vertical tubes to locate a standard xcex3-ray source. The source and a detector are moved simultaneously by a step-by-step motor, each step occurring after a pre-specified number of pulses have been delivered to the detector. The time interval between successive steps is proportional to the water content of the snow therebetween.
U.S. Pat. No. 3,843,887, Self-Calibrating Isotopic Precipitation Measurement Gage, to Morrison, Oct. 22, 1974, uses an isotopic source of xcex3-rays transmitted through attenuating material to a given point at which detectors are located. Processors associated with the detectors establish a continuously calibrated density measurement from which the SWE is calculated.
U.S. Pat. No. 4,047,042, Method of Determining the Water Equivalent Depth of Snowfall Using Neutrons of Cosmic Rays, to Wada et al., Sep. 6, 1977, provides a method of determining SWE from the count of survival neutrons of cosmic rays that have passed through a pre-specified depth of snow.
U.S. Pat. No. 5,594,250, Method for Detecting Water Equivalent of Snow Using Secondary Cosmic Gamma Radiation, to Condreva, Jan. 14, 1997, provides a remotely activated, battery-powered system that transmits SWE measurements derived from the attenuation of secondary xcex3-radiation.
U.S. Pat. No. 6,313,645 B1, Method of Determining the Volumetric Proportion of Liquid Water and the Density of Snow and a Device for Carrying Out the Method, to Brandelik, Nov. 6, 2001, describes a probe having three parallel electrical conductors that are energized in pairs. Knowing dielectric coefficients, based on probe-specific data and known coefficients of snow and water and energizing at differing frequencies, volumetric parts of water, snow, and air in the location are determined using the law of mass conservation.
Some patents describe methods of determining water content using thermal sensors. U.S. Pat. No. 4,886,088, Method and Apparatus for Measuring Water Content, to Ryokai et al., Dec. 12, 1989, describes a method for measuring thermal conductivity of a target and comparing that with known data to establish a threshold indicating sufficient water content in the target.
U.S. Pat. No. 5,287,734, Thermal Sensor, to Cuming, Feb. 22, 1994, details a method for determining soil moisture levels using thermal diffusivity techniques. It uses a heated sensor and a reference sensor together with a comparator circuit to determine relative moisture based on comparing measurements from the two sensors.
Other patents describe systems used to provide warning of geodetic perturbations. U.S. Pat. No. 5,177,471, Landslide Detecting Apparatus Having Improved Optical Unit, to Horibati et al., Jan. 5, 1993, describes an optical device that warns of impending landslides. The apparatus responds to pressure changes in the earth""s crust by shading parallel beams of light upon movement of the earth""s crust.
U.S. Pat. No. 5,661,464, Snow Pack Stability Monitor, to Bilak et al., Aug. 26, 1997, provides a probe consisting of a movable and stationary part. When the movable part moves in relation to the stationary part, a motion detector senses this and telemeters details of the movement to a central station.
U.S. Patent Application Publication No. 2002/0060576 A1, by Tominaga, May 23, 2002, details an active electrical probe consisting of four electrodes inserted into the soil. Soil conductivity is measured under known conditions and changes in conductivity are sensed by the apparatus and data sent to a central station to effect appropriate warning.
Patents have issued on underground water level sensors, deployed in arrays, an example of which is U.S. Pat. No. 6,119,535, Underground Water Level Sensing Unit and Ground Monitoring System Using the Same, to Tambo et al., Sep. 19, 2000. Each sensing unit in the array employs a gyro-sensor that responds to an external force such as movement of a snowpack or topsoil. The data thus detected are telemetered to a central station for initiating appropriate warnings.
As can be inferred from the above descriptions, the embodiments proposed for use with the present invention are much simpler and less expensive to implement, while yielding wholly satisfactory results.
A preferred embodiment of the present invention accurately measures SWE while eliminating the need for fluid-filled pillows that contain environmentally hazardous fluids. It also minimizes effects of thermal processes at the snow/ground interface that cause SWE pressure sensor measurement errors. Further, it minimizes thermal effects by keeping soil moisture under the detector the same as in the soil immediately surrounding the detector, matching the thermal conductivity of the SWE detector with the surrounding soil while exhibiting a relatively low specific heat.
In general, a system is provided that accurately determines relative pressure at a boundary between first and second media. It uses at least one detector incorporating structure for mounting and enclosing at least one sensor such that at least a portion of the structure is in contact with one of the two media of interest for which the detector is interposed between. This structure enhances the thermal stability of the detector as does a cover disposed over the detector and positioned between the detector and the one media that does not contact the detector. This system maintains thermal conductivity comparable to its surrounding environs. Sensors used with the system to measure pressure may be: load cells, strain gauges, displacement sensors using lasers that measure deflection, displacement sensors using capacitance to measure deflection, and combinations thereof. Further, the detector has a low aspect ratios i.e., its thickness is less than about three percent of its longest dimension. Materials having low specific heat values of less than about 0.1 cal/gm-K at 20xc2x0 C. are used in its construction. These low specific heat materials may be: aluminum, stainless steel, low specific heat synthetic material, low specific heat plastics, and combinations thereof. Further the detector may include a layer of heat diffusing material internal thereto that may be any of: elastomeric material, metal, polymer foam, a honeycomb configuration fabricated from metal, a honeycomb configuration fabricated from a synthetic material, and combinations thereof. To improve performance the detector may comprise or be covered with a material that assists in matching its thermal response to its surroundings. The material used for the cover may be any of: a moisture impermeable membrane, a moisture permeable membrane, a synthetic material, a layer of soil of approximately 5 cm thickness, and combinations of the above. The structure of the detector is such that it enables an effective Young""s Modulus at least twenty-five percent greater than the Young""s Modulus of its surrounding environs.
In summary, a preferred embodiment of the present invention provides a detector that approximately matches its thermal response to the thermal response of its surrounding environs, thus facilitating accurate measurement of pressure difference between two abutting media. This is particularly suited to for establishing accurate estimates of snow water equivalent.
Another embodiment envisions a monitoring system that provides accurate estimates of relative pressure from data collected at a boundary of first and second media. It incorporates at least one detector having a structure for mounting and enclosing at least one sensor so that at least a portion of the structure is in contact with one of the media and interposed between them. The structure itself enhances the thermal stability of the detector which is further augmented by the addition of a cover, disposed over the structure so as to be positioned between the structure and the media that does not contact it. To facilitate use of the data collected by the detector, there may be provided a data transceiver associated with the detector for communicating data and receiving information that may be used to control the handoff of data; a data collector for receiving the data at a place possibly remote from the detector; and a processor for manipulating the data for further uses, such as planning, warning, or input to scientific studies.
In one embodiment, an electronic detector system determines the pressure of overlying material, such as snow or soil overburden, and may be used to determine the amount of moisture stored therein, e.g., the snow water equivalent (SWE). The detector system minimizes measurement errors that occur when thermal properties of the detector, i.e., the sensor and its housing, itself are different from those of its surrounds. Primary features of the detector are:
a thin, wide geometry, i.e., a low aspect ratio, to minimize stress concentration effects associated with detector geometry and differences in the mechanical properties of the detector and its surrounding environment. Taylor, Donald W., Review of Pressure Distribution Theories, Earth Pressure Cell Investigations, and Pressure Distribution Data, prepared for the U.S. Army Corps of Engineers under Contract No. W22-015 eng-185, Massachusetts Institute of Technology, Cambridge, Mass, 1945;
inherent low heat capacity to minimize buildup or storage of heat;
thermal conductivity close to that of its environs to minimize differences in heat flow through the detector compared to the adjacent environs;
a membrane cover, for use with an impermeable version, that is at least three times the diameter, or width of the impermeable detector, to maintain appropriate soil moisture and soil thermal conductivity around the detector;
an alternative permeable design to allow water to flow through the detector to the underlying environs to maintain appropriate soil moisture and thermal conductivity in the immediate vicinity of the permeable detector;
a heat diffusing layer above the detector to evenly distribute heat or cold over and around the detector to minimize differences in heat flux through the detector as compared to immediately surrounding environs;
a thin profile to reduce the influence of any thermal conductivity differences between the detector and its immediate environs; and
a detector incorporating an electronic transducer that eliminates a need for conventional fluid-filled pillows and associated plumbing.
The detector""s relatively low specific heat limits the amount of heat capable of storage in the detector. This minimizes the differential rate of snowmelt on the ground compared to the SWE detector in early winter before snow cover is established.
Assuring the thermal conductivity of the detector approximates the thermal conductivity of its environs, e.g., approximately 0.8-2 watts/Kelvin-meter (W/K-m) for soil, results in comparable heat flux through the detector and its surrounding environs. For example, snow will melt at about the same rate whether it is over the detector or over its immediate surrounding environs. This minimizes the transfer of snow load onto or off of the detector, reducing measurement errors.
Using a waterproof membrane for an impermeable, or non-perforated, detector to maintain approximately the same moisture in the environment under and around the detector is accomplished by adding a cover membrane with a diameter or width at least three times that of the detector plates. This keeps the thermal conductivity of the environment under and around the detector equivalent to that of the detector. Using an alternative design having permeable, or perforated, top and bottom load support plates allows moisture to flow through the permeable detector to maintain comparable moisture under and around the detector, thus maintaining comparable thermal conductivity of the environment adjacent the detector. This is especially important during spring runoff when the amount of snowmelt water varies diurnally. The thermal conductivity of the soil is a strong function of its water content and maintaining comparable water content of the environment under and around the detector assures comparable conductivity.
Using a thin detector, e.g.,  less than 2.5 cm (1 in.), improves performance since the small thickness of the detector decreases the impact of the difference between the thermal conductivity of the detector and its surrounding environment. The limitation on detector thickness is a factor of the technology employed as well as cost.
Employing a heat diffusing layer over the detector that extends beyond the edge of the detector by at least the diameter or width of the detector minimizes any difference in heat flux between the detector and its surrounds. The diffusing layer may be soil, a synthetic, or the impermeable membrane used with the impermeable detector configuration.
Features of the detector that are important to its performance, but are not particularly novel since they are well known from soil stress sensor theory or previous snow pillow experiments are:
To maintain a small aspect ratio (i.e., the ratio of the detector thickness to its width or diameter). This ensures that the detector""s stress concentration factor has a magnitude close to one and that differences in the elastic modulus of the detector compared to the soil and snow have little effect on the measurement.
To construct the detector with an effective elastic (Young""s) modulus that is significantly higher than the surrounding material (snow). This ensures that the detector""s deformation is related to the snow pressure (stress sensor) rather than the snow deformation (strain sensor).
To use electronic methods to determine the SWE, instead of fluid-filled pressure sensors. Electronic methods include load cells, strain gauges, displacement sensors (laser, capacitance, etc.). Electronic measurement methods eliminate the possibility of environmentally hazardous antifreezes from entering the environment.
To construct the detector with a diameter designed to minimize SWE errors associated with different snow melt rates on the detector and surrounding soil. This diameter depends on how well the thermal properties of the detector are matched to the soil. If they are well matched, the detector diameter can be quite small. If, however, the detector""s thermal properties are not well matched to the soil, as is the case for fluid-filled snow pillows, the diameter, or width, may need to be quite large. For example, total diameter of CRREL test SWE detector is 1 m (3.3 ft) with an active load area of only 0.46 m (1.5 ft). The diameter of a standard snow pillow is typically 3 m (10 ft).
The SWE electronic detector is an improvement over the fluid-filled snow pillow because the invention uses electronic load measuring methods, is smaller and lighter than the snow pillow, and accommodates the physical effects of thermal processes that affect SWE measurements. It operates with significantly lower measurement error than a snow pillow because it is thermally matched to the surrounding soil. As a result, the detector may be packaged in a smaller housing than the fluid-filled snow pillow while recording SWE with high accuracy. It has a low heat capacity compared to the snow pillow because of the materials used, e.g., aluminum, other metal, or low heat capacity synthetics, as compared to the snow pillow with its high heat capacity fluid-filled bladder. Differences in the thermal conductivity of the surrounding soil and the detector are minimized by matching the thermal conductivity of the detector to the midrange value of thermal conductivity for soils, i.e., approximately 0.8-2 W/K-m, since soil thermal conductivity depends strongly on its moisture content. This is accomplished by constructing the detector with either perforated top and bottom plates or covering the detector and the surrounding soil with an impermeable membrane that has a diameter at least three times greater than the solid (un-perforated) detector plates. Both configurations maintain the soil moisture near the detector at near constant values, reducing the variation of thermal conductivity in the soil near the detector compared to the soil farther away. The permeable detector has the added advantage of keeping soil moisture levels near their natural levels. This cannot be done using impermeable detectors such as a snow pillow. In addition, the SWE electronic detector makes measurements electronically so it does not require the extensive plumbing needed to deploy a fluid-filled snow pillow. Finally, there is no possibility of leaking antifreeze into the soil.
The electronic SWE detector responds instantly to changes in SWE. This capability is useful for making stream runoff estimates, and is an improvement over the gamma ray attenuation method that requires a three to four hour averaging time for measurements.
The detector measures the SWE of wet or dry snow. This is a capability not available with microwave methods that are useful only with dry snow. Compared to any of the conventional methods, the detector is less sensitive to changes in thermal conditions of the soil.
Additionally, the perforated configuration of the electronic SWE detector may be used in applications where maintaining a water flow through soil is important or if there is a need for a separate measurement of the overburden soil load from fluid pressure in the soil.